Numerous secondary oil recovery projects use carbon dioxide as a miscible injectant to recover additional oil from existing oil reservoirs. Carbon dioxide has the ability to reduce the surface tension of oil adhering to shale or rocks, causing it to flow out of the oil well. As the oil is produced from the carbon dioxide injection, the related gas produced will increase in volume and carbon dioxide concentration. The total gas volume will increase by a factor of five to seven. The hydrocarbon gas volume will increase by a factor of two. The carbon dioxide concentration will increase from 5 to 90 percent by volume over the nine to ten year life of the project. The composition of the hydrocarbons will vary little.
This carbon dioxide must be removed before the hydrocarbons can be utilized as a product. The recovered carbon dioxide is reinjected into the wells. A number of process methods have been developed and are being tried for this purpose. They include the conventional amine absorption systems, seloxol absorption and many others. These conventional process schemes have proven to be very expensive and energy inefficient when compared to recently developed cryogenic distillative process methods such as the process taught by the U.S. Pat. No. 4,462,814, issued July 31, 1984, to Arthur S. Holmes and James M. Ryan, entitled "Distillative Separation of Gas Mixtures Containing Methane, carbon dioxide and Other Components", known in the industry as the Ryan/Holmes process. This process teaches a method of separating the hydrocarbons from the carbon dioxide and a method of preventing azeotrope formation of carbon dioxide and ethane during the distillation separation process.
The distillation process schemes utilized in attempting to separate methane from the carbon dioxide and other hydrocarbons must deal with the potential formation of carbon dioxide solids. Ryan/Holmes solves this problem by introducing an agent to prevent the formation of solid carbon dioxide. A second problem that distillation process methods must handle is the separation of ethane from carbon dioxide due to the formation of the carbon dioxide-ethane azeotrope. This azeotrope formation limits the degree to which the carbon dioxide may be separated from the ethane. The Ryan/Holmes process solves this problem by the further introduction of the aforementioned agent. This agent is a propane-and-heavier mixture of hydrocarbons or some other hydrocarbon miscible non-polar liquid. This agent serves to maintain the relative volatility of carbon dioxide to ethane above one, and the azeotrope is not formed.
For further illustrations of carbon dioxide removal, see U.S. Pat. No. 4,475,347, issued Oct. 9, 1984, to Hegarty et al., entitled "Process for Separating carbon dioxide and Sulphur-Containing Gases from a Synthetic Fuel Production Process Off-Gas"; U.S. Pat. No. 3,453,835, issued July 8, 1969, to Hochgesand, entitled "Absorption of carbon dioxide Employing Separately Cooled Absorbent Streams"; U.S. Pat. No. 3,977,203, issued Aug. 31, 1976, to Hinton et al., entitled "Purification of Natural Gas by Liquid/Liquid Extraction with a Polar Solvent"; U.S. Pat. No. 4,252,548, issued Feb. 24, 1981, to Breiter et al., entitled "carbon dioxide Removal from Methane-Containing Gases"; U.S. Pat. No. 4,293,322, issued Oct. 6, 1981, to Ryan et al., entitled "Distillative Separation of carbon dioxide from Hydrogen Sulfide": U.S. Pat. No. 4,318,723, issued Mar. 9, 1982, to Holmes et al., entitled "Cryogenic Distillative Separation of Acid Gases from Methane"; U.S. Pat. No. 4,350,511, issued Sept. 21, 1982, to Holmes et al., entitled "Distillative Separation of carbon dioxide from Light Hydrocarbons": U.S. Pat. No. 4,383,841, issued May 17, 1983, to Ryan et al., entitled "Distillative Separation of carbon dioxide from Hydrogen Sulfide"; U.S. Pat. No. 4,383,842, issued May 17, 1983, to 0'Brien, entitles "Distillative Separation of Methane and carbon dioxide"; U.S. Pat. No. 4,462,814, issued July 31, 1984, to Holmes et al., entitled "Distillative Separation of Gas Mixtures Containing Methane, carbon dioxide and Other Components"; U.S. Pat. No. 3,595,782, issued July 27, 1971, to Bucklin et al., entitled "Method for Separating carbon dioxide from Hydrocarbons"; U.S. Pat. NO. 3,640,052, issued Feb. 8, 1972, to Konoki et al., entitled "Process for Removing carbon dioxide in a Combined System for Producing Ammonia and Urea"; U.S. Pat. No. 3,683,634, issued Aug. 15, 1972, to Streich, entitled "Fractionation with Subsequent Recombination if Feed in Double Column Rectifier"; U.S. Pat. No. 3,899,212, issued Aug. 12, 1975, to Kruis et al., entitled "Extraction of Odorizing Sulfur Compounds from Natural Gas and Reodorization Therewith"; U.S. Pat. No. 3,983,711, issued Oct. 5, 1976, to Solomon, entitled "Plural Stage Distillation of a Natural Gas Stream"; U.S. Pat. No. 4,097,250, issued June 27, 1978, to Pagani et al., entitled "Method for the Purification of Natural Gas Having a High Contents of Acidic Acid"; U.S. Pat. No. 4,115,086, issued Sept. 19, 1978, to Jordan et al., entitled "Recovery of Light Hydrocarbons from Refinery Gas". U.S. Pat. No. 4,149,864, issued Apr. 16, 1979, to Eakman et al., entitled "Separation of carbon dioxide and Other Acid Gas Components from Hydrocarbon Feed"; U.S. Pat. No. 4,152,129, issued May 1, 1979, to Trentham et al., entitled "Method for Separating carbon dioxide from Methane"; U.S. Pat. No. 4,185,978, issued Jan. 29, 1980, to McGallard et al., entitled "Method for Cryogenic Separation of carbon dioxide from Hydrocarbons"; and U.S. Pat. No. 4,311,495, issued Jan. 19, 1982, to Stryino, entitled "Separating carbon dioxide and ethane by Liquid-Liquid Extraction."